Enable timeout on reading from socket in safekeeper WAL service.

TCP_KEEPALIVE is not enabled by default, so this prevents hanged up connections
in case of abrupt client termination. Add 'closed' flag to PostgresBackendReader
and pass it during handles join to prevent attempts to read from socket if we
errored out previously -- now with timeouts this is a common situation.

It looks like
2023-04-10T18:08:37.493448Z  INFO {cid=68}:WAL
receiver{ttid=59f91ad4e821ab374f9ccdf918da3a85/16438f99d61572c72f0c7b0ed772785d}:
terminated: timed out

Presumably fixes https://github.com/neondatabase/neon/issues/3971

libs/postgres_backend/src/lib.rs

@@ -54,7 +54,7 @@ pub fn is_expected_io_error(e: &io::Error) -> bool {
     use io::ErrorKind::*;
     matches!(
         e.kind(),
-        ConnectionRefused | ConnectionAborted | ConnectionReset
+        ConnectionRefused | ConnectionAborted | ConnectionReset | TimedOut
     )
 }
 
@@ -320,9 +320,17 @@ impl<IO: AsyncRead + AsyncWrite + Unpin> PostgresBackend<IO> {
         if let ProtoState::Closed = self.state {
             Ok(None)
         } else {
-            let m = self.framed.read_message().await?;
-            trace!("read msg {:?}", m);
-            Ok(m)
+            match self.framed.read_message().await {
+                Ok(m) => {
+                    trace!("read msg {:?}", m);
+                    Ok(m)
+                }
+                Err(e) => {
+                    // remember not to try to read anymore
+                    self.state = ProtoState::Closed;
+                    Err(e)
+                }
+            }
         }
     }
 
@@ -493,7 +501,10 @@ impl<IO: AsyncRead + AsyncWrite + Unpin> PostgresBackend<IO> {
             MaybeWriteOnly::Full(framed) => {
                 let (reader, writer) = framed.split();
                 self.framed = MaybeWriteOnly::WriteOnly(writer);
-                Ok(PostgresBackendReader(reader))
+                Ok(PostgresBackendReader {
+                    reader,
+                    closed: false,
+                })
             }
             MaybeWriteOnly::WriteOnly(_) => {
                 anyhow::bail!("PostgresBackend is already split")
@@ -510,8 +521,12 @@ impl<IO: AsyncRead + AsyncWrite + Unpin> PostgresBackend<IO> {
                 anyhow::bail!("PostgresBackend is not split")
             }
             MaybeWriteOnly::WriteOnly(writer) => {
-                let joined = Framed::unsplit(reader.0, writer);
+                let joined = Framed::unsplit(reader.reader, writer);
                 self.framed = MaybeWriteOnly::Full(joined);
+                // if reader encountered connection error, do not attempt reading anymore
+                if reader.closed {
+                    self.state = ProtoState::Closed;
+                }
                 Ok(())
             }
             MaybeWriteOnly::Broken => panic!("unsplit on framed in invalid state"),
@@ -797,15 +812,25 @@ impl<IO: AsyncRead + AsyncWrite + Unpin> PostgresBackend<IO> {
     }
 }
 
-pub struct PostgresBackendReader<IO>(FramedReader<MaybeTlsStream<IO>>);
+pub struct PostgresBackendReader<IO> {
+    reader: FramedReader<MaybeTlsStream<IO>>,
+    closed: bool, // true if received error closing the connection
+}
 
 impl<IO: AsyncRead + AsyncWrite + Unpin> PostgresBackendReader<IO> {
     /// Read full message or return None if connection is cleanly closed with no
     /// unprocessed data.
     pub async fn read_message(&mut self) -> Result<Option<FeMessage>, ConnectionError> {
-        let m = self.0.read_message().await?;
-        trace!("read msg {:?}", m);
-        Ok(m)
+        match self.reader.read_message().await {
+            Ok(m) => {
+                trace!("read msg {:?}", m);
+                Ok(m)
+            }
+            Err(e) => {
+                self.closed = true;
+                Err(e)
+            }
+        }
     }
 
     /// Get CopyData contents of the next message in COPY stream or error
@@ -923,7 +948,7 @@ pub enum CopyStreamHandlerEnd {
     #[error("EOF on COPY stream")]
     EOF,
     /// The connection was lost
-    #[error(transparent)]
+    #[error("connection error: {0}")]
     Disconnected(#[from] ConnectionError),
     /// Some other error
     #[error(transparent)]